1. Field of the Invention
This invention relates to circuit breakers of the type having a bimetallic thermal trip element and, more particularly, to a circuit breaker for use in distribution transformers to control moderate power distribution on feeder circuits having a bimetallic trip element wherein the power dissipation is concentrated in the base of the bimetal.
2. Description of the Prior Art
The disclosed circuit breaker is particularly adaptable for use with distribution transformers. Transformers used in power distribution systems are generally associated with a protective device which prevents or limits current overload damage to the transformer and its associated apparatus. The completely self-protected transformer includes a circuit breaker on the secondary or low voltage side to protect against damage due to overload currents. The secondary breaker disconnects the transformer from its load if the load current becomes dangerously high.
Commonly used circuit breakers incorporate three basic features: (1) a low overload signal device, (2) an incremental increase adjustment and (3) a tripping device to open the contacts of the circuit breaker upon a predetermined overload. A bimetallic element is disposed in series in the circuit breaker. As the load current increases through the circuit breaker, a low overload point is reached at which the bimetallic element deflects enough to activate a signal light on the outside of the transformer housing. The signal light which is mounted on the transformer provides a visual indication that the secondary circuit breaker is about to trip. That is, the signal light is turned on at a lower overload than that required to trip the circuit breaker, thereby indicating that the load current is approaching trip level. As the load current continues to increase, the bimetallic element deflects further until a second overload point is reached at which the circuit breaker trips open. The circuit breaker tripping operation protects the transformer against severe damage due to the flow of excessive overload current. The bimetallic element is also responsive to the ambient temperature of the surrounding oil allowing a predetermined oil temperature rise to also produce the required deflection to activate the signal light or trip the breaker. It is desirable to minimize the power dissipated in the bimetallic element during operation while still achieving the required deflections. The bimetallic elements utilized in prior art circuit breakers are generally constructed for a linear power dissipation along their lengths.